This disclosure relates generally to packages for lighting devices that incorporate light-emitting diodes (LEDs) and in particular to a package for flip-chip LEDs with close spacing of LED chips.
Etendue quantifies the size and angular spread of a beam of light. In the case of a light source, etendue is calculated by multiplying the area of the light source and the solid angle of the emitted light beam. Etendue is subject to the following conservation principle: given a particular light source, etendue cannot be decreased without incurring light loss.
In some contexts, such as stage lighting, low etendue is particularly desirable because lighting devices with low etendue provide more control over the lighting of particular portions of the stage. Low etendue, combined with the ability to control the color and brightness of individual lighting devices, allows production designers to achieve desired effects, such as spotlighting particular actors or portions of the set, illuminating different areas in different colors, etc.
LED-based lighting technology offers the prospect of energy-efficient and customizable theater lighting. An LED (or “LED chip”), as used herein, refers to a light-emitting diode, i.e., a semiconductor device that emits light in response to electrical current. LEDs typically emit narrow-band light, with a central frequency that depends on the particular construction of the LED. At present, LEDs are commercially available at various wavelengths spanning the visible-light spectrum from red to violet; infrared and ultraviolet LEDs are also commercially available. LED chips are typically fabricated to include a light-emitting surface (which may be identified as the “top” surface) and at least two metal bonding pads, which are typically disposed on the top and/or bottom surfaces of the LED chip. In some cases, some or all of the light-emitting surface of the LED may be coated with a wavelength-shifting material (e.g., phosphor) that shifts some of the light emitted by the LED to a longer wavelength. For instance, a “white” LED can be created by applying yellow phosphor material to a blue LED chip or by other processes that provide light of a combination of wavelengths.
In application, LEDs are typically packaged into structures referred to as “emitters.” As used herein, an “emitter” refers to a structure that includes one or more LEDs and additional structures that provide mechanical and electrical support for the LED(s) as well as transfer of heat away from the LED(s). For instance, an emitter typically includes a substrate (e.g., a ceramic substrate) that may be patterned with electrical contacts. The LED(s) can be mounted on the substrate and bonded to some of the electrical contacts; other electrical conducts may be used to connect the emitter to a current source to drive the LED(s). An emitter may also include a cover disposed over the LED(s) to protect the LED(s) from the elements while allowing light to escape. The cover can be optically transparent and may incorporate wavelength-shifting elements, focusing or defocusing elements (e.g., a convex or concave surface that provides lensing behavior) diffusive elements, or the like. An emitter typically provides exposed electrical contacts that can be connected to an external power source to deliver operating current to the LED(s).
Existing emitters are generally not optimal for theater lighting and other contexts where low entendue is desired. For example, to produce a bright enough light for use in a theater context typically requires that the light source include a large number of LEDs, and the requirements of thermal and electrical performance may limit the density of LEDs in an emitter. For a given brightness, limits on the density of LEDs translate into a lower limit on the area of the light source, which may undesirably increase etendue. Accordingly, LED-based lighting devices with reduced etendue would be desirable.